/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* _reverseList(ListNode* cur, ListNode* tail)
    {
        if (cur == nullptr)
            return tail;

        ListNode* tmp = cur->next;
        cur->next = tail;

        return _reverseList(tmp, cur);
    }

    ListNode* reverseList(ListNode* head) {
        return _reverseList(head, nullptr);
    }
};